1. Field of the Invention
The present invention relates to a CVCF (Constant Voltage Constant Frequency) inverter and to a method for controlling the same, and particularly to a CVCF inverter employed in carrying out lap switching with a rotating type power generator.
2. Description of the Prior Art
In a large aircraft in flight, for example, electric power is supplied from a rotating type power generator which is driven by an engine. While on land, on the other hand, the power supply is switched from the power generator on the aircraft to a power supply installed at an airport. In this case, uninterruptible power switching is required because of computers installed in the aircraft. A CVCF inverter is generally used with the power supply installed at the airport.
FIG. 1 shows a conventional power supply system of this type. In FIG. 1, reference numeral 30 designates a rotating type power generator (MG) on the aircraft; 40 designates a CVCF inverter (CVCF); 50 designates apparatuses (loads) on the aircraft to which the power is to be supplied; SW1 and SW2 designates switches; .DELTA.I designates a cross current; and IL designates a load current. The switch SW1 is closed in flight so that the power is supplied from the power generator 30 to the loads 50. After landing, uninterruptible switching is carried out by first closing the switch SW2 so that both switches SW1 and SW2 are closed for a while, and then, by opening the switch SW1 so that the loads 50 are supplied with the power from the CVCF inverter 40 via the switch SW2. Thus, the lap switching operation is performed.
The lap switching operation, however, may sometimes cause a cross current .DELTA.I due to a difference in phases of the voltages supplied from the power generator 30 and the CVCF inverter 40. More specifically, assuming that the output voltage and the output impedance of the CVCF inverter 40 are Asin.omega.t and Z1, and those of the power generator 30 are Asin(.omega.t+.theta.) and Z2, a cross current .DELTA.I expressed by equation (1) flows from the power generator 30 to the CVCF inverter 40. EQU .DELTA.I=A{sin (.omega.t+.theta.)-sin.omega.t}/(Z1+Z2) EQU =2A cos (.omega.t+.theta./2) sin (.theta./2)/(Z1+Z2) (1)
Thus, the magnitude of the cross current .DELTA.I is directly proportional to the phase difference .theta. as long as .theta. is rather small.
Although the cross current presents little problem as long as the phase difference .theta. is small, a large phase difference will result in a large current flowing from the power generator 30 to the CVCF inverter 40 or from the CVCF inverter 40 to the power generator 30. For example, when the phase of the power generator 30 leads that of the CVCF inverter 40, the cross current flows from the power generator 30 to the CVCF inverter 40 so that the DC voltage of the CVCF inverter 40 rises, which might cause damage to devices in the inverter in the worst case.
On the other hand, when the phase of the CVCF inverter 40 leads that of the power generator 30, the cross current flows from the CVCF inverter 40 to the power generator 30. In this case, if the output current of the CVCF inverter 40 exceeds a limiting level of an overcurrent, automatic control to limit the output voltage of the CVCF inverter 40 is carried out to protect the inverter 40 from damage. This creates another problem in that sufficient power is not supplied because of the voltage drop at the loads.